Nucleic acid amplification techniques such as polymerase chain reaction (PCR), ligase chain reaction (LCR), strand displacement amplification (SDA), and self-sustained sequence replication (3SR) have had a major impact on molecular biology research. In particular, PCR, although a relatively new technology, has found extensive application in various fields including the diagnosis of genetic disorders, the detection of nucleic acid sequences of pathogenic organisms in clinical samples, the genetic identification of forensic samples, and the analysis of mutations in activated oncogenes. In addition, PCR amplification is being used to carry out a variety of tasks in molecular cloning and analysis of DNA. These tasks include the generation of specific sequences of DNA for cloning or use as probes, the detection of segments of DNA for genetic mapping, the detection and analysis of expressed sequences by amplification of particular segments of cDNA, the generation of libraries of cDNA from small amounts of mRNA, the generation of large amounts of DNA for sequencing, the analysis of mutations, and for chromosome crawling. During the next few years, PCR, other amplification methods, and related technologies are likely to find increasing application in many other aspects of molecular biology.
Unfortunately, problems exist in the application of PCR to clinical diagnostics. Development has been slow due in part to: labor intensive methods for detecting PCR product; susceptibility of PCR to carryover contamination—false positives due to contamination of a sample with molecules amplified in a previous PCR; and difficulty using PCR to quantitate the number of target nucleic acid molecules in a sample. A need exists for a simple method of quantitative analysis of target nucleic acid molecules in a sample.
Recently, significant progress has been made in overcoming some of the problems of clinical diagnostic nucleic acid amplification through the development of automatable assays for amplified product that do not require that the reaction vessel be opened, thereby minimizing the risk of carryover contamination. Most of these assays rely on changes in fluorescent light emission consequent to hybridization of a fluorescent probe or probes to amplified nucleic acid. One such assay involves the hybridization of two probes to adjacent positions on the target nucleic acid. The probes are labeled with different fluors with the property that energy transfer from one fluor stimulates emissions from the other when they are brought together by hybridization to adjacent sites on the target molecule.
Another assay, which is commercially available, is the “TaqMan” fluorescence energy transfer assay and kit, available from Perkin Elmer, Applied Biosystems Division, Foster City, Calif. This type of assay is disclosed in the publication of Holland et al., Detection of specific polymerase chain reaction product by utilizing the 5′→3′ exonuclease activity of Thermus aquaticus DNA polymerase, Proc. Natl. Acad. Sci. USA, Vol. 88, pp. 7276–7280, August 1991, and in the publication of Livak et al., Oligonucleotides with Fluorescent Dyes at Opposite Ends Provide a Quenched Probe System Useful for Detecting PCR Product and Nucleic Acid Hybridization, PCR Methods and Applic., 4, pp. 357–362 (1995). The “TaqMan” or 5′ exonuclease assay uses a single nucleic acid probe complementary to the amplified DNA and labeled with two fluors, one of which quenches the other. If PCR product is made, the probe becomes susceptible to degradation via an exonuclease activity of Taq polymerase that is specific for DNA hybridized to template (“TaqMan” activity). Nucleolytic degradation allows the two fluors to separate in solution which reduces quenching and increases the intensity of emitted light of a certain wavelength. Because these assays involve fluorescence measurements that can be performed without opening the amplification vessel, the risk of carryover contamination is greatly reduced. Furthermore, the assays are not labor intensive and are easily automated.
The TaqMan and related assays have provided new ways of quantitating target nucleic acids. Early methods for quantitation relied on setting up amplification reactions with known numbers of target nucleic acid molecules and comparing the amount of product generated from these control reactions to that generated from an unknown sample, as reviewed in the publication by Sninsky et al. The application of quantitative polymerase chain reaction to therapeutic monitoring, AIDS 7 (SUPPL. 2), PP. S29–S33 (1993). Later versions of this method used an “internal control”, i.e., a target nucleic acid added to the amplification reaction that should amplify at the same rate as the unknown but which could be distinguished from it by virtue of a small sequence difference, for example, a small insertion or deletion or a change that led to the gain or loss of a restriction site or reactivity with a special hybridization probe, as disclosed in the publication by Becker-Andre, et al., Absolute mRNA quantification using the polymerase chain reaction (PCR). A novel approach by a PCR aided transcript titration assay (PATTY), Nucleic Acids Res., Vol. 17, No. 22, pp. 9437–9446 (1989), and in the publication of Gilliland et al., Analysis of cytokine mRNA and DNA: Detection and quantitation by competitive polymerase chain reaction, Proc. Natl. Acad. Sci. USA, Vol. 87, pp. 2725–2729 (1990). These methods have the disadvantage that slight differences in amplification efficiency between the control and experimental nucleic acids can lead to large differences in the amounts of their products after the million-fold amplification characteristic of PCR and related technologies, and it is difficult to determine relative amplification rates accurately.
Newer quantitative PCR methods use the number of cycles needed to reach a threshold amount of PCR product as a measure of the initial concentration of target nucleic acid, with DNA dyes such as ethidium bromide or SYBRTM Green I, or “TaqMan” or related fluorescence assays used to follow the amount of PCR product accumulated in real time. Measurements using ethidium bromide are disclosed in the publication of Higuchi et al., Simultaneous Amplification and Detection of Specific DNA Sequences, BIO/TECHNOLOGY, Vol. 10, pp. 413–417 (1992). “TaqMan” assays used to follow the amount of PCR product accumulated in real time are disclosed in the publication of Heid et al., Real Time Ouantitative PCR, Genome Research, Vol. 6, pp. 986–994 (1996), and in the publication of Gibson et al., A Novel Method for Real Time Ouantitative RT-PCR, Genome Research, Vol. 6, pp. 995–1001 (1996). However, these assays also require assumptions about relative amplification efficiency in different samples during the exponential phase of PCR.
An alternative method of quantitation is to determine the smallest amount of sample that yields PCR product, relying on the fact that PCR can detect a single template molecule. Knowing the average volume of sample or sample dilution that contains a single target molecule, one can calculate the concentration of such molecules in the starting sample. However, to accumulate detectable amounts of product from a single starting template molecule usually requires that two or more sequential PCRs have to be performed, often using nested sets of primers, and this accentuates problems with carryover contamination.
Careful consideration of the factors affecting sensitivity to detect single starting molecules suggests that decreasing the volume of the amplification reaction might improve sensitivity. For example, the “TaqMan” assay requires near saturating amounts of PCR product to detect enhanced fluorescence. PCRs normally saturate at about 1011 product molecules/microliter (molecules/μl) due in part to rapid reannealing of product strands. To reach this concentration of product after 30 cycles in a 10 μl PCR requires at least 103 starting template molecules (103×230/10 μl≈1011/μl). Somewhat less than this number of starting molecules can be detected by increasing the number of cycles, and in special circumstances even single starting molecules may be detectable as described in the publication of Gerard et al., A Rapid and Ouantitative Assay to Estimate Gene Transfer into Retrovirally Transduced Hematopoietic Stem/Progenitor Cells Using a 96-Well Format PCR and Fluorescent Detection System Universal for MMLV-Based Proviruses, Human Gene Therapy, Vol. 7, pp. 343–354 (1996). However, this strategy usually fails before getting to the limit of detecting single starting molecules due to the appearance of artifactual amplicons derived from the primers (so called “primer-dimers”) which interfere with amplification of the desired product.
If the volume of the PCR were reduced 1000-fold to ˜10 nanoliters (nl), then a single round of 30 cycles of PCR might suffice to generate the saturating concentration of product needed for detection by the TaqMan assay, i.e. 1×230 per 10 nanoliters ≈1011 per microliter. Attempts have been made to miniaturize PCR assemblies but no one has developed a cost-effective PCR assembly which can carry out PCR in a nanoliter-sized sample. Part of the problem with miniaturization is that evaporation occurs very rapidly with small sample volumes, and this problem is made worse by the need to heat samples to ˜90° C. during thermocycling.
In addition to potential advantages stemming from ability to detect single target nucleic acid molecules, miniaturization might also facilitate the performance of multiple different amplification reactions on the same sample. In many situations it would be desirable to test for the presence of multiple target nucleic acid sequences in a starting sample. For example, it may be desirable to test for the presence of multiple different viruses such as HIV-1, HIV-2, HTLV-1, HBV and HCV in a clinical specimen; or it may be desirable to screen for the presence of any of several different sequence variants in microbial nucleic acid associated with resistance to various therapeutic drugs; or it may be desirable to screen DNA or RNA from a single individual for sequence variants associated with different mutations in the same or different genes, or for sequence variants that serve as “markers” for the inheritance of different chromosomal segments from a parent. Amplification of different nucleic acid sequences and/or detection of different sequence variants usually requires separate amplification reactions with different sets of primers and/or probes. If different primer/probe sets were positioned in an array format so that each small region of a reaction substrate performed a different amplification/detection reaction, it is possible that multiple reactions could be carried out in parallel, economizing on time, reagents, and volume of clinical specimen.
A need therefore exists for a device that can form and retain a sample volume of about 10 nanoliters or less and enable amplification to be performed without significant evaporation. A need also exists for a reliable means of detecting a single starting target nucleic acid molecule to facilitate quantification of target nucleic acid molecules. A need also exists for performing multiple different amplification and detection reactions in parallel on a single specimen and for economizing usage of reagents in the process.